1. Field of the Invention
The invention concerns a method and an apparatus for magnetic resonance imaging. In particular, the invention concerns such a method and such an apparatus in which a transverse magnetization can be specifically dephased using one or more gradient pulses in preparation of the magnetization.
2. Description of the Prior Art
Magnetic resonance (MR) imaging is an imaging modality that is used for examination and diagnosis in many fields of medicine. It based on the physical effect of nuclear magnetic resonance. To acquire MR signals, a defined longitudinal magnetization is generated or suppressed in a preparation module or multiple preparation modules of an imaging sequence. The dynamics of the nuclear spins can be observed after radiating radio-frequency pulses.
In MR imaging, the suppression of unwanted signal contributions is gaining high importance. For example, in the clinical routine, signal contributions from specific regions should regularly be suppressed in order to reduce motion or pulsation artifacts that (for example) result from breathing or blood flow, in particular in arteries. Alternatively or additionally, the imaging should be accelerated, for example by reducing the observation region (“Field of View”, FOV), or by suppression of outer image regions (“Outer Volume Suppression”, OVS). Moreover, methods should also be used in which specific spin types can be selectively suppressed that have an exceptional chemical shift. For example, a suppression of signal portions of fat, water or silicon can be achieved in this manner.
One or more radio-frequency (RF) excitation pulses with which the signal to be suppressed is specifically excited—i.e. is converted from longitudinal magnetization into transversal magnetization—can be used for signal suppression. Alternatively or additionally, a gradient pulse or multiple gradient pulses can be used in order to dephase the transverse magnetization so that the transverse magnetization delivers a strongly reduced contribution or no contribution in the following imaging modules of the sequence. This is also designated as “gradient spoiling” or “spoiling”.
Imaging sequences are known in which measures to suppress specific signal contributions are implemented in chronological sequence in different preparation modules, but such measures can lead to unwanted coherence paths. The interaction of the spoiler gradient fields or spoiler RF fields that are used in different preparation modules can lead to an unwanted rephasing of the signal contributions that should actually be suppressed. A semi-classical theoretical description of such an unwanted rephasing for gradient fields that are spatially variable only in a predetermined spatial direction and are generated during the imaging is provided by K. Scheffler in “A Pictorial Description of Steady-States in Rapid Magnetic Resonance Imaging”, Concepts in Magnetic Resonance 11, 291-304 (1999).
Unwanted signal contributions can also occur due to non-ideal conditions in the contrast preparation. For example, some imaging methods use RF pulses for preparation of a saturation contrast or an inversion contrast. The longitudinal magnetization that is used for imaging is initially saturated or inverted and signals caused thereby are acquired with one or more imaging modules after a defined wait time. The saturation can take place by excitation by an RF pulse and subsequent dephasing. The inversion can take place by the application of an RF inversion pulse. In such methods as well, it is desirable to suppress unwanted signal coherence paths.
Various techniques can be used to suppress unwanted coherence paths. For example, a technique known as RF spoiling is known. An exemplary method to suppress unwanted signal coherences via RF spoiling is described by Y. Zur in “Spoiling of Transverse Magnetization in Steady-State Sequences”, MRM 21, 251-263 (1991). RF spoiling is suitable to effectively suppress the unwanted coherences of RF pulses generated in short and constant time intervals. However, RF spoiling can be difficult to apply in certain methods or can deliver an insufficient suppression of unwanted coherence paths in which only a few preparation modules are used. Examples of such methods are saturation or contrast preparation methods.
Gradient spoiling can be alternatively or additionally used. Gradient spoiling is well suited to dephasing selectively generated transversal magnetizations. A conventional gradient spoiling in which the same gradient moment of the spoiler gradient field is always used in different preparation modules can lead, however, to unwanted coherence paths as described above. An exemplary method for suppression of unwanted signal coherences by gradient spoiling is described by H. Z. Wang and S. J. Riederer in “A Spoiling Sequence for Suppression of Residual Transverse Magnetization”, MRM 15, 175-191 (1990). Although such semi-empirical methods suppress unwanted signal coherences, they can lead to a rapid growth of the necessary gradient moments. For example, a doubling of the gradient for a specific, predetermined direction takes place in H. Z. Wang and S. J. Riederer, “A Spoiling Sequence for Suppression of Residual Transverse Magnetization”, MRM 15, 175-191 (1990). Given a large number of preparation modules, it is possible that the required large gradient moments can no longer be generated, or they may entail severe limitations in the temporal progression of the measurement sequence.
DE 10 2009 019 895 B4 describes a method and a device for diffusion-weighted acquisition of MR signals in which gradient spoiling is used. The spoiler gradient fields spatially vary along one direction.